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Asphalt Material Design Inputs for Use with the Mechanistic-Empirical Pavement Design Guide in Virginia

The Guide for the Mechanistic-Empirical Design of New & Rehabilitated Pavement Structures (MEPDG), developed under NCHRP Project 1-37A and recently adopted by the American Association of State Highway and Transportation Officials (AASHTO), offers an improved methodology for pavement design and evaluation. To achieve this improved prediction capability, the MEPDG procedure requires fundamental material properties in addition to certain empirically determined binder and mixture properties as design inputs. One of the key tasks identified by the Virginia Department of Transportation’s (VDOT) Asphalt Concrete MEPDG Committee was the laboratory characterization of asphalt mixtures commonly used in Virginia to generate a catalog of the MEPDG-required design inputs.

The purpose of this study was to evaluate, compile, and present asphalt material properties in a format that could be readily used in the MEPDG software and to develop a comprehensive catalog of MEPDG design input parameters for pavement design in Virginia. To achieve this objective, 18 asphalt concrete mixtures, sampled from seven of the nine VDOT districts, were tested using a battery of MEPDG- required tests including dynamic modulus (|E*|), flow number (FN), creep compliance, tensile strength, and beam fatigue tests. Testing involving binder and volumetric properties of the mixtures was also conducted. Finally, rut tests using the asphalt pavement analyzer (APA), a standard VDOT test protocol, were conducted to enable a direct comparison of the APA and FN test results. On the basis of these tests, suggestions for additional studies were made.

The results of the study were presented in a form matching the MEPDG input format, and a catalog of design input parameters was developed for the 18 asphalt concrete mixtures. Included in the catalog were binder stiffness, mixture |E*|, mixture gradation, and mixture volumetric properties that would enable a designer the flexibility to select the desired input level (1, 2, or 3) depending on the pavement type. An illustrative example of how the developed inputs could be implemented using the MEPDG software was also provided. The results showed that |E*| master curves of asphalt mixtures obtained using the five standard testing temperatures described in AASHTO TP 62 could be obtained by testing at only three temperatures, which could result in a substantial reduction of testing time. The results also showed that the FN test was a sensitive test for evaluating rutting susceptibility of asphalt mixtures in the laboratory. The FN test was found to be sensitive to binder stiffness, mixture stiffness, mixture volumetric properties, aggregate gradation, and amount of recycled asphalt pavement (RAP) for the mixtures considered in this study.

The study recommends that the catalog of input data for typical asphalt mixtures developed in this study be considered for pavement design in Virginia. The data followed expected trends and compared quite well with those reported in previous studies. Further studies should be conducted to evaluate the FN test as an additional tool for evaluating rutting in asphalt mixtures. Mixtures containing higher amounts of RAP (>20%) exhibited comparatively lower rutting resistance than those with 20% or less RAP. This phenomenon was unexpected since it is generally believed that adding more RAP should result in stiffer and hence more rut-resistant mixtures. Additional research should be conducted to investigate this phenomenon further.